Mu opiate receptors are the principal brain sites for the analgesic, euphoric and addicting activities of morphine and heroin. Pharmacologic data suggest that these receptors contain binding sites for agonists and antagonists, including opiate "small molecule" drugs and larger neuropeptides, but details of the means whereby these receptor proteins recognize these ligands are currently unknown. In the current FY, mutated, deleted, and chimeric versions of the rat and human mu receptors have been created and studied to refine our understanding of receptor/ligand interactions, and of receptor intrinsic activities and G-protein coupling. N-terminal involvement in binding appeared minimal; 64 N-terminal amino acids of the rat mu receptor could be removed without effects on radioligand binding. Further deletion of 33 C-terminal residues yielded a receptor which recognized mu-selective agonists morphine and DAMGO ([D-Ala2,MePhe4,Gly-ol5]enkephalin) at wild type levels. Three charged transmembrane residues of the mu receptor, Asp-114, Asp-147 and His-297, were critical for high affinity agonist recognition, suggesting possible ionc interactions with aspects of mu receptor radioligands. Results of studying chimeric mu/kappa receptors in which the putative second extracellular loop of the wild type mu receptor has been replaced with that of the kappa receptor fit with a substantial role for this extracellular loop in dynorphin peptide recognition. These studies substantially enhance our working knowledge of means whereby opiate receptors recognize opiate drugs and opioid peptides, and provide significant clues to new pathways for formulation of selective new drugs with activities at these receptors.